It is the aim of all controlled release preparations to provide a longer duration of pharmacological response after the administration of the dosage form than is ordinarily experienced after the administration of an immediate release dosage form. Such extended periods of response provides for many inherent therapeutic benefits that are not achieved with short acting, immediate release products.
Controlled release formulations known in the art include specially coated beads or pellets, coated tablets and ion exchange resins, wherein the slow release of the active drug is brought about through selective breakdown of, or permeation through, the coating of the preparation or through formulation with a special matrix to affect the release of the drug.
An important aspect of all forms of controlled release dosage forms is related to the stability of the same. The stability of a pharmaceutical dosage form is related to maintaining its physical, chemical, microbiological, therapeutic, pharmaceutical, and toxicological properties when stored, i.e., in a particular container and environment. Stability study requirements are covered, e.g., in the Good Manufacturing Practices (GMPs), the U.S.P, as well as in New Drug Applications (NDAs) and Investigational New Drug Applications (INDs).
The ingredients used in sustained release dosage formulations often present special problems with regard to their physical stability during storage. For example, waxes which have been used in such formulations are known to undergo physical alterations on prolonged standing, thus precautions are taken to stabilize them at the time of manufacture or to prevent the change from occurring. Fats and waxy materials when used in purified states are known to crystallize in unstable forms, causing unpredictable variations in availability rates during stability testing at the time of manufacture and during later storage.
It is known that certain strategies can be undertaken to obtain stabilized controlled release formulations in many cases, such as insuring that the individual ingredients are in a stable form before they are incorporated into the product, and that processing does not change this condition, retarding the instability by including additional additives, and inducing the individual ingredients of the dosage form to reach a stable state before the product is finally completed.
It is also recognised that the moisture content of the product can also influence the stability of the product. Changes in the porosity and/or hydration level of a polymeric film, such as the ethyl celluloses, can alter the rate of water permeation and drug availability. Also, binders such as acacia are known to become less soluble when exposed to moisture and heat. Such problems have been handled by controls in the processing method and proper packaging of the product.
Hydrophobic polymers such as certain cellulose derivatives, zein, acrylic resins, waxes, higher aliphatic alcohols, and polylactic and polyglycolic acids have been used in the prior art to develop controlled release dosage forms. Methods of using these polymers to develop controlled release dosage forms such as tablets, capsules, suppositories, spheroids, beads or microspheres are to overcoat the individual dosage units with these hydrophobic polymers. It is known in the prior art that these hydrophobic coatings can be applied either from a solution, suspension or dry. Since most of these polymers have a low solubility in water, they are usually applied by dissolving the polymer in an organic solvent and spraying the solution onto the individual drug forms (such as beads or tablets) and evaporating off the solvent.
Aqueous dispersions of hydrophobic polymers have been used in the prior art to coat pharmaceutical dosage forms for aesthetic reasons such as film coating tablets or beads or for taste-making. However, these dosage forms are used for immediate release administration of the active drug contained in the dosage form.
The use of organic solvents in the preparation of polymer coatings is considered problematic as the formulations have inherent problems with regard to flammability, carcinogenicity, and safety in general. In addition, the use of organic solvents is disfavored due to environmental concerns.
Therefore, it is desirable to prepare a controlled release formulation prepared from an aqueous dispersion of a hydrophobic polymer. However, to date, attempts to prepare stable controlled release pharmaceutical formulations using aqueous dispersions of hydrophobic polymers have been unsuccessful due to stability problems. In particular, when coating these pharmaceutical forms using aqueous polymeric dispersions to obtain a desired release profile of the active drug(s) over several hours or longer, it is known in the art that the dissolution release profile changes on ageing. It is also known that this instability problem does not exist when the polymers are applied from organic solvent solution.
For example, Dressman, et al., Proceed. Intern. Symp. Control. Rel. Bioact. Mater., 18 (1991), pp. 654-655, Controlled Release Society, Inc. reported on tests conducted which showed that phenylpropanolamine HCl pellets coated with an ethyl cellulose-based film are only stable at room temperature under ambient humidity conditions. In these experiments, phenylpropanolamine HCl was overlaid on sugar seeds to a 76% loading, and coated with 10% ethyl cellulose applied from an aqueous dispersion. A second sample consisted of phenylpropanolamine spheronized with microcrystalline cellulose in a 70:30 ratio, then coated with 15% ethyl cellulose applied from an aqueous dispersion. Samples from each batch were stored for up to four weeks under conditions of room temperature/ambient humidity; room temperature/high humidity (75% RH); 37.degree. C./ambient humidity; and 37.degree. C./high humidity. The data for the dissolution profiles indicated that the lag time and percent drug released at 8 hours were unstable at all conditions other than room temperature/ambient humidity conditions.
Although the authors considered the pellets to be unaffected by storage conditions, they concluded that the release mechanism from the phenylpropanolamine HCl pellets overcoated with ethyl cellulose-based films appear to depend upon the pellet composition, and that under high relative humidity storage, the rate of release may be effected, especially if the samples were stored at elevated temperature.
Munday, et al., Drug Devel. and Indus. Phar., 17 (15) 2135-2143 (1991) report that film coated theophylline mini-tablets film coated with ethyl cellulose with PEG (2:1), and ethyl cellulose with Eudragit L (2:1) proved to have impeded dissolution upon storage under stress conditions, the degree of slowdown of release being said to be directly proportional to temperature, while the effect of relative humidity (RH) appeared to be insignificant.
The authors concluded therein that the decreased rate of release was due to the slowing in the rate of molecular diffusion of the drug across the polymeric coating material, and suggested that the change was due to significant alterations in the permeability of the polymer which occurred during the experimental storage.
Aqueous polymeric dispersions have been used to produce stable controlled release dosage forms, but this has only been possible by other methods such as incorporation of the same into the matrix of the dosage form, rather than via the use of a coating of the aqueous polymeric dispersion to obtain retardant properties.